The structures of the hematologic or hematopoietic system include the blood, blood vessels, and blood-forming organs (bone marrow, spleen, liver, lymph nodes, and thymus gland). The major function of blood is to carry necessary materials (oxygen, nutrients) to cells and to remove carbon dioxide and metabolic waste products. The hematologic system also plays an important role in hormone transport, the inflammatory and immune responses, temperature regulation, fluid-electrolyte balance, and acid-base balance.

Bone Marrow

Contained inside all bones, occupies interior of spongy bones and center of long bones; collectively one of the largest organs of the body (4%-5% of total body weight)

Primary function is hematopoiesis (the formation of blood cells)

Two kinds of bone marrow, red and yellow

Red (functioning) marrow

Carries out hematopoiesis; production site of erythroid, myeloid, and thrombocytic components of blood; one source of lymphocytes and macrophages

Found in ribs, vertebral column, other flat bones

Yellow marrow: red marrow that has changed to fat; found in long bones; does not contribute to hematopoiesis

All blood cells start as stem cells in the bone marrow; these mature into the different, specific types of cells, collectively referred to as formed elements of blood or blood components: erythrocytes, leukocytes, and thrombocytes.

Blood

Composed of plasma (55%) and cellular components (45%)

Hematocrit

Reflects portion of blood composed of red blood cells

Centrifugation of blood results in separation into top layer of plasma, middle layer of leukocytes and platelets, and bottom layer of erythrocytes.

Majority of formed elements is erythrocytes; volume of leukocytes and platelets is negligible.

Albumin: largest of plasma proteins, involved in regulation of intravascular plasma volume and maintenance of osmotic pressure

Serum globulins: alpha, beta, gamma

Alpha: role in transport of steroids, lipids, bilirubin

Beta: role in transport of iron and copper

Gamma: role in immune response, function of antibodies

Fibrinogen, prothrombin, plasminogen (see Coagulation)

Cellular ComponentsCellular components or formed elements of blood are erythrocytes (red blood cells [RBCs]), which are responsible for oxygen transport; leukocytes (white blood cells [WBCs]), which play a major role in defense against microorganisms; and thrombocytes (platelets), which function in hemostasis.

Erythrocytes carry antigens, which determine the different blood groups.

Blood-typing systems are based on the many possible antigens, but the most important are the antigens of the ABO and Rh blood groups because they are most likely to be involved in transfusion reactions.

ABO typing

Antigens of system are labelled A and B.

Absence of both antigens results in type O blood.

Presence of both antigens is type AB.

Presence of either A or B results in type A and type B respectively.

Nearly half the population is type O, the universal donor.

Antibodies are automatically formed against the ABO antigens not on person's own RBCs; transfusion with mismatched or incompatible blood results in a transfusion reaction (Table 4.17).

1%-2% of red cell mass or 200 ml blood/minute stored in spleen; blood comes via the splenic artery to the pulp for cleansing, then passes into splenic venules that are lined with phagocytic cells, and finally to the splenic vein to the liver.

Important hematopoietic site in fetus; postnatally produces lymphocytes and monocytes

Important in phagocytosis; removes misshapen erythrocytes, unwanted parts of erythrocytes

Also involved in antibody production by plasma cells and iron metabolism (iron released from Hgb portion of destroyed erythrocytes returned to bone marrow)

In the adult, functions of the spleen can be taken over by the reticuloendothelial system.

LiverSee also Gastrointestinal Tract.

Involved in bile production (via erythrocyte destruction and bilirubin production) and erythropoiesis (during fetal life and when bone marrow production is insufficient).